1. Field of the Invention
The invention relates to the use of liposomes in the drug delivery system, and more particularly to the stable liposomes capable of incorporating high content of hydrophobic drugs.
2. Description of the Related Art
Liposome technology has been exploited extensively for the purpose of drug delivery for many years. A typical liposome structure is composed of single or multiple layer membranes with hydrophobic domain between the phospholipid bilayers, and the interior aqueous compartment. Hydrophobic or hydrophilic compounds can be entrapped in the hydrophobic domain or encapsulated in the aqueous compartment, respectively. On the other hand, liposomes can be constructed of natural constituents so that the liposome membrane is in principal identical to the lipid portion of natural cell membranes. It is considered that liposomes are quite compatible with the human body when used as drug delivery system. In addition, liposome-based drug formulation also has been reported to be able to achieve the equivalent therapeutic efficacy to free drug, as well as reduce the systemic toxicity in many applications.
The hydrophobic drug, paclitaxel, was sold in the market in 1992, and used in phase II trials for treating breast and ovarian cancer. In 1998, it was used in combination therapy with cisplatin for the treatment of non-small cell lung and ovarian cancer in phase I trials. However, due to its poor solubility in water, paclitaxel is prepared for clinical administration containing Cremophor EL® (polyethoxylated castor oil) and absolute ethanol in a 50/50 (vol/vol) ratio (Diluent 12). In clinical trials, the problems of anaphylactoid reaction, neutropenia, peripheral neuropathy, bradyarrhythmia and anemia were encountered. Meanwhile, the amount of cremophor EL necessary to solubilize the clinically required dose of paclitaxel is much higher than that administered with any other marketed drug. Cremophor vehicle thus is found to be responsible for hypersensitivity response. Premedication with corticosteroid, diphenhydramine or H2 antagonist, and slow infusion of a large volume are needed to avoid the side effect. In contrast, owing to the aforementioned advantages of liposome-based drug delivery system, researches of incorporating paclitaxel in liposomes for clinical paclitaxel administration have become a hot topic and been reported regularly.
Conventional paclitaxel-liposomes were prepared at paclitaxel/lipid molar ratio of approximately 3 mole % regardless of whether the liposomes are made of a mixture of phosphatidyl glycerol (PG) and phosphatidyl choline (PC) (U.S. Pat. No. 5,415,869; Sampedro, F et al., J Micrencapsul 11:309-318 (1993); Sharma, A. et al., Pharm Res 11:889-896 (1994); Shien, M. F. et al., J Ferm Bioeng 83:87-90 (1997)), or of unsaturated (U.S. Pat. No. 6,090,955; Bartoli, M. H. et al., J Micrencapsul 7:191-197 (1990); Riondel, J. et al., In Vivo 6:23-28 (1992); Sharma, D. et al., Melanoma Res 8:240-244 (1998)) or partially unsaturated PC (U.S. Pat. No. 5,683,715). At a drug/lipid ratio of 4 mole %, the paclitaxel-liposome system is stable only for 2 days while needle-like crystal precipitates appear during preparation at a drug/lipid ratio up to 8 mole % (Sathyamangalam, V. et al., Biochemistry 33:8941-8947 (1994); Bernsdorff, C. et al., J Biomed Mater Res 46:141-149 (1999)). On the other hand, the liposomes are prepared by employing hydrophilic polymer-conjugated phospholipid (methoxy polyethylene glycol-phosphatidyl ethanolamine) in order to enhance its circulation time in blood post iv administration (Crosasso, P. et al., J Control Release 63:19-30 (2000)). Liposomes with the prolonged circulation time in bloodstream make it possible increasing the availability of the injected liposomes to reach the target cells before being metabolized. However, this formulation of the polymer-engrafted liposomes with a maximal 3 mole % (paclitaxel/lipid ratio) quickly become unstable in one-week storage at 4° C.
Alternatively, a formulation of paclitaxel-liposomes comprising a special phospholipid, cardiolipid, and phosphatidyl choline (PC) was disclosed in U.S. Pat. No. 5,424,073 and Int J Oncol 12:1035-1040 (Cabanes, A. et al., 1998). The molecular structure of cardiolipid is composed of one huge hydrophilic head and four aliphatic chains. The liposomes prepared in accordance with this formulation increase the paclitaxel/lipid molar ratio to 9 mole %, however, it is stable only for 1 month when stored in liquid form at 4° C.
Generally, paclitaxel incorporated within the bilayer membrane of liposomes is thermodynamically prone to self-aggregation, then precipitating from liposomes. Previous researches have reported that the optimal paclitaxel/lipid molar ratio in a typical liposome formulation is ranged from 3 to 4 mole %, and paclitaxel-liposomes are more stable when the drug/lipid ratio is kept at approximately 3 mole %. When the molar ratio is increased, needle-like crystal precipitates appear during the preparation process. Besides, it is known by person skilled in the art that drugs with a low drug/lipid ratio are commonly unsuitable for clinical administration. A high dose of liposomes still may result in certain extent of toxicity due to the injection of excessive amounts of lipids in the body. Furthermore, increasing liposome concentration also raises the cost of production. Therefore, it is important to elevate the hydrophobic drug/lipid ratio in liposome-based drug delivery system by which the above drawbacks may be avoided.